This function seems not to work at all for me (4.0 RC1). I had to modify box width in pixels in Layout/CSS, which is probably overriding that option:

#joomlacommentform textarea {

    width: 600px;/*400px;*/

    height: 150px;/*200px;*/

    border: 1px solid #999999;/*#CCCCCC;*/

    background-color: #BABABA; /*zmieniono*/

}